CSS-only — this section uses only CSS for the corners.
Should work in all non-IE browsers, rounding the corners correctly.
IEv9, when in "IEv9 standards" mode should also work.
Untested in IEv10, but I suspect it will work there similar to IEv9.
If you're not using IE, PIE won't be used, so these will match the above!
PIE — this section uses PIE in IE for the corners.
Should work in all IE browsers, rounding the corners correctly, mostly.
Note that while the "darken" (left) hover effect is rounded, the opacity is lost.
Also, while the area is rounded, the hover effect is not on the "lighten" (right) image.
Mostly PIE — this section uses PIE in IE for the corners.
The "darken" item regains its opacity, but loses its rounded corners (on hover).
This is accomplished by not including the translucent "background" in the PIE directive in the JS.